This application claims priority from Korean Patent Application Nos. 10-2003-0013532 and 10-2004-0008652 filed on Mar. 4, 2003 and Feb. 10, 2004, respectively, in the Korean Intellectual Property Office, the disclosures of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a reliable broadcast system and method thereof operated under an ad-hoc network environment and, more particularly, to a system and method for reliably broadcasting a data packet under an ad-hoc network environment, thereby decreasing any loss of data packets as well as an overload of the entire system under the ad-hoc network environment.
2. Description of the Related Art
Generally, mobile ad-hoc networks serve to provide multi-hop based communication services with mobile terminals connected by wireless links, whereby it is possible to configure a network between wireless terminals under a non-infrastructure environment.
Due to the inherent nature of the wireless multi-hop, these mobile ad-hoc networks have a wide interference area wherein mobile nodes overlap. Within these overlapping areas which there is a high probability of generating a collision between data in the air during data transmission/reception. To avoid such a collision, a channel reservation mechanism is used based on the procedure of RTS (request-to-send)-CTS (clear-to-send)-data-ACK (acknowledgement), which is used only for unicast, but not used for broadcast.
With use of the channel reservation mechanism for the broadcast, the channel reservation mechanism must be applied to all the respective neighboring nodes. Hence, when data passes through only to one adjacent hop consisting of a plurality of nodes, a time equal to [the number of neighboring nodes * T(RTS-CTS-data-ACK)] is consumed. In the worst case, this time is increased by a value equal to (1+collision times) {the number of neighboring nodes * T(RTS-CTS-data-ACK)}.
Because the channel reservation mechanism is not used for broadcasting on the mobile ad-hoc network, the probability of the data collision in the air becomes higher, thereby deteriorating the reliability of data transmission.
When an ACK-based mechanism is used to solve this problem, ACKs must be received from all neighboring nodes. Thus, the minimum time equal to [the number of neighboring nodes * T(ACK)] is consumed only for receiving the ACKs.
FIG. 1 shows a general mobile ad-hoc network environment, under which mobile nodes are connected by wireless links without any infrastructure.
As shown in FIG. 1, in the mobile ad-hoc network, a network topology is changed frequently due to the mobility of the nodes constituting the network. Therefore, in order to identify states of neighboring nodes as well as the topology of the entire network, the nodes of the mobile ad-hoc network periodically exchange Hello packets containing both information of their own Internet protocols (IPs) and the identified neighboring nodes.
FIG. 2 shows a process by which each node broadcasts under a conventional mobile ad-hoc network environment, wherein when an arbitrary node Ni intends to broadcast data to all nodes within the network, a flooding approach for relaying the data via neighboring nodes must be used.
Specifically, when an arbitrary sender node Ni (e.g. node N3) broadcasts the data to all nodes within the network, the data which the sender node Ni transmits, as shown, are transmitted to the neighboring nodes of the sender node Ni first (from N3 to N1 and N4). Then, each of the neighboring nodes receiving the data to be broadcast transmits the data to its neighboring nodes again (from N1 to N2, N6 and N7, and from N4 to N5 and N6). Again, each of the neighboring nodes transmits the data to its neighboring nodes. Thus, this transmission procedure allows the data transmitted from the sender node N1 to be transmitted to all the nodes within the network.
Respective nodes are connected by wireless links using an omni antenna, thereby causing the nodes to overlap between a transmission area and a reception area. For instance, the transmission area of the node N3 overlaps transmission areas of the nodes N1 and N4, and also the transmission area of the node N1 overlaps transmission areas of the nodes N2, N6 and N7.
Therefore, in order to broadcast the data with no collision, the data must be continuously transmitted between the nodes having the overlapped transmission area according to a specific timing (see FIG. 3). However, it is impossible to adjust the timing in the mobile ad-hoc network because the mobile ad-hoc network does not make use of the infrastructure.
In this manner, the unmatched timing between the whole nodes results in generating a collision between the data during broadcasting. After the collision, data transmission is delayed as shown in FIG. 4. Ultimately, some nodes fail to receive the broadcast data.
FIG. 5 shows another process by which each node broadcasts under a conventional mobile ad-hoc network environment.
Here, an approach of transmitting ACKs is used to determine whether or not the broadcast data is transmitted again, thereby preventing a situation where some nodes do not receive the broadcast data. Because the ACKs must be received from all neighboring nodes, the time required to broadcast the data results in an increase of a unit time in proportion to the number of the neighboring nodes whenever a data packet moves from one hop to another hop, so that the use of the ACKs causes the data transmission to be greatly delayed. Further, to avoid a collision between the ACKs during their transmission, which may result from the overlap between transmission and reception areas of the nodes, the ACKs must be transmitted after the timing is adjusted at each node, i.e., they must be continuously transmitted (see FIG. 6).